Antibodies to Liver Sinusoidal Endothelial Cells Modulate Immune Responses in Liver Transplantation

Liver immunology

The liver is the largest organ in the body and is generally regarded by nonimmunologists as having little or no lymphoid function. However, such is far from accurate. This review highlights the importance of the liver as a lymphoid organ. Firstly, we discuss experimental data surrounding the role of liver as a lymphoid organ. The liver facilitates tolerance rather than immunoreactivity, which protects the host from antigenic overload of dietary components and drugs derived from the gut and it is instrumental to fetal immune tolerance. Loss of liver tolerance leads to autoaggressive phenomena, which if not controlled by regulatory lymphoid populations, may lead to the induction of autoimmune liver diseases. Liver-related lymphoid subpopulations also act as critical antigen-presenting cells. The study of the immunological properties of liver and delineation of the microenvironment of the intrahepatic milieu in normal and diseased livers provides a platform to understand the hierarchy of a series of detrimental events that lead to immune-mediated destruction of the liver and the rejection of liver allografts. The majority of emphasis within this review will be on the normal mononuclear cell composition of the liver. However, within this context, we will discuss selected, but not all, immune-mediated liver disease and attempt to place these data in the context of human autoimmunity.

Participation of blood vessel cells in human adaptive immune responses. Trends Immunol. 2012;33:49-57. [PMID: 22030237 DOI: 10.1016/j.it.2011.09.006]

Circulating T cells contact blood vessels either when they extravasate across the walls of microvessels into inflamed tissues or when they enter into the walls of larger vessels in inflammatory diseases such as atherosclerosis. The blood vessel wall is largely composed of three cell types: endothelial cells lining the entire vascular tree; pericytes supporting the endothelium of microvessels; and smooth muscle cells forming the bulk of large vessel walls. Each of these cell types interacts with and alters the behavior of infiltrating T cells in different ways, making these cells active participants in the processes of immune-mediated inflammation. In this review, we compare and contrast what is known about the nature of these interactions in humans.

Analysis of time-course gene expression profiles of sinusoidal endothelial cells during liver regeneration in rats

Liver regeneration (LR) after partial hepatectomy (PH) requires the coordinate contribution of different cell types. Liver sinusoidal endothelial cells (LSECs), representing the largest population of nonparenchymal cells, are proven to be crucial in LR. However, the details about their implications in regeneration are not still clear. In this study, percoll density centrifugation and immunomagentic bead methods were used to isolate LSECs with high purity and yield; global transcriptional profiles of LSECs during the regeneration were investigated by microarray. 1,629 genes were identified to be LR-related. Among them, there were 833 known genes whose expression patterns were clustered into eight classes. Gene function enrichment analysis showed that genes involved in the major LSEC functions, i.e., coagulation, phagocytosis, and transport, were highly enriched in cluster characterized by rapid induction and gradual return, suggesting the quick reestablishment of LSEC function after PH. Genes in immunity/inflammation and defense response were enriched in clusters exhibiting transient downregulation and quick recovery, possibly being associated with suppression of immunity/inflammation pathway in LSECs at early phase. Genes in glycogen synthesis and glycolysis were enriched in the clusters marked by "significant increase and gradual return" and "slight increase and then downregulation", implying an enhanced carbohydrate metabolism at early phase; detoxification-related genes were markedly distributed in the cluster with feature of rapid increase and then reduction, which was helpful in eliminating waste substance. Taken together, the measurement of gene expression profiling of LSECs and expression pattern analysis of functionally categorized genes gave insight into the mechanism of action of this cell on LR.

In hepatic fibrosis, liver sinusoidal endothelial cells acquire enhanced immunogenicity

The normal liver is characterized by immunologic tolerance. Primary mediators of hepatic immune tolerance are liver sinusoidal endothelial cells (LSECs). LSECs block adaptive immunogenic responses to Ag and induce the generation of T regulatory cells. Hepatic fibrosis is characterized by both intense intrahepatic inflammation and altered hepatic immunity. We postulated that, in liver fibrosis, a reversal of LSEC function from tolerogenic to proinflammatory and immunogenic may contribute to both the heightened inflammatory milieu and altered intrahepatic immunity. We found that, after fibrotic liver injury from hepatotoxins, LSECs become highly proinflammatory and secrete an array of cytokines and chemokines. In addition, LSECs gain enhanced capacity to capture Ag and induce T cell proliferation. Similarly, unlike LSECs in normal livers, in fibrosis, LSECs do not veto dendritic cell priming of T cells. Furthermore, whereas in normal livers, LSECs are active in the generation of T regulatory cells, in hepatic fibrosis LSECs induce an immunogenic T cell phenotype capable of enhancing endogenous CTLs and generating potent de novo CTL responses. Moreover, depletion of LSECs from fibrotic liver cultures mitigates the proinflammatory milieu characteristic of hepatic fibrosis. Our findings offer a critical understanding of the role of LSECs in modulating intrahepatic immunity and inflammation in fibro-inflammatory liver disease.

Evidence for no relevance of anti-major histocompatibility complex class I-related chain a antibodies in liver transplantation

The polymorphic major histocompatibility complex class I-related chain A (MICA) antigen is being increasingly recognized as a potential target molecule for immune cells during allograft rejection. Here we studied whether MICA is a target antigen for antibodies in liver transplant patients. Eighty-four patients were investigated for the presence of MICA antibodies before and after liver transplantation with MICA-transfected cells and flow cytometry. MICA typing was performed by polymerase chain reaction. Expression of MICA in liver cells was determined by reverse-transcription polymerase chain reaction, Western blotting, and flow cytometry. Liver biopsy specimens from liver transplant patients were examined for MICA expression. A total of 22 of 84 (26%) patients had MICA antibodies either pre-transplant (8/84, 9.5%) or post-transplant (14/84, 17%). No correlation between rejection frequencies (14/22, 63%) or other clinical parameters was observed in patients with MICA antibody versus those without MICA antibody (29/62, 47% P = not significant). We found weak messenger RNA expression for MICA in liver cells but no protein or cell surface expression. In addition, no MICA expression in liver biopsy sections from liver transplant patients was observed at any time point, including rejections. Thus, our preliminary results demonstrate no causal relationship between the presence of MICA antibodies and liver allograft rejections. Therefore, it is likely that MICA may not be an important target antigen during liver allograft rejections.

Relevance of MICA and other non-HLA antibodies in clinical transplantation

The clinical importance of HLA-specific antibodies for organ allograft outcome is well established. In the past few years, there has been an increasing interest in non-HLA antigens as targets of injury in organ transplant recipients. This increased interest has been spurred by the fact that HLA-identical kidney transplants also undergo immunological rejections. Polymorphisms within non-HLA genes associated with evoking an immune response to alloantigens are currently being studied for their association with transplant outcome. Non-HLA antigens, such as the polymorphic MHC class I-related chain A (MICA), expressed on endothelial cells have been implicated in the pathogenesis of hyperacute, acute and chronic organ allograft rejections. Use of endothelial cells as targets may clarify the specificities of other clinically relevant non-HLA antibodies in graft rejections. This review summarizes past and current knowledge of the clinical importance and specificities of non-HLA antibodies, and mechanisms by which these antibodies may contribute to graft destruction in clinical transplantation. The aims of current research into the role of non-HLA antigens and their genetics in predicting outcome are to develop an improved insight into the basic science of transplantation and to develop a risk or prognostic index for use in the clinical setting. Non-HLA antibody responses are receiving increasing interest in acute and chronic rejection and specificity, affinity, and pathogenicity need to be investigated to estimate their contribution. Undoubtedly, this will continue to be an area of interest in terms of fully understanding the role of non-HLA antigens as targets of immune-mediated injury and the potential for clinical intervention.